The present invention relates to a power conversion device that converts DC power to AC power, as well as to a power conversion control method for the same.
The isolation transformer for grid tie applications is driven at a low frequency of several dozen Hz, which is the same as that of the grid, so there has been a problem that it is difficult to reduce the size and weight of the transformer. In recent years, the use of solid state transformer (SST) for high voltage and high power application has been studied. SST is the technology that replaces the conventional transformer by a combined configuration of a high frequency transformer and a power circuit such as DC/DC converter and inverter for driving the high frequency transformer to output AC of the same frequency as the conventional one at the output or input. By driving the high frequency transformer at a high frequency of several dozen to several hundred kHz, it is possible to significantly reduce its size and weight as compared to a single conventional transformer, even with the configuration of SST in which the power circuit is added.
The installation of solar electricity generation as power applications for grids has grown extensively worldwide, and there is a demand for a high performance power conditioning system (PCS) that controls the power of solar electricity generation and outputs to the grid. FIG. 2 shows a configuration of a conventional PCS for large power output. The PCS is a system with DC power of solar electricity generation as the input, converting DC to three-phase AC power by a three-phase inverter, increasing the three-phase AC voltage that is output from the PCS to a level from several hundred V to 6.6 kV or more by the use of a step-up transformer, and outputting to the grid. In order to output the maximum power of solar electricity generation to the grid, the PCS implements MPPT control. Since the voltage of solar power generation by the MPPT control varies significantly according to the weather, a step-up chopper is placed between the three-phase inverter and the solar power generation in order to handle the input voltage variation due to the MPPT control.
Japanese Unexamined Patent Application Publication No. 2001-209445 describes a grid-tie inverter device that maintains the operation frequency of a step-up converter approximately constant, even if the voltage of the DC input power supply is changed, to prevent an increase in noise and loss.